@Dragou
It’s neither theory nor spoilers. It’s the main difference between the manga and the novel.
At Chapter 1 in the manga it was explained that the avatar is a base human that you can “attach” accessories that change your avatar’s appearance, like equipping “elf ears” to become an elf and the like. Arc paid for a full body avatar of a skeleton.
In the novel on the other hand, it wasn’t fully explained in the beginning and was later found out through the hot springs that lifted a curse that he was a dark elf. His internal thoughts then went on to say that he bought a skeleton skin that he layered over his dark elf avatar.
This is one of the main differences between the novel and the manga. The “curse” in the novel didn’t become resistant to the spring and instead he had to keep drinking it to alleviate the effects to change from a skeleton whenever those situations arise where he has take off his helmet to show his face. The problem is the effects faded over time unless he took another drink.
Edit: Here in the manga, it’s just to help dampen the burden of his main body from the accumulated suppressed emotions.
He also already claimed he was an elven knight in this chapter, right? They only ever agreed that he can use Lalatoia for his last name, and not necessarily pretend to be an elf. In the novel, however, it was because they discovered he was an elf. So the manga author may have skimped over the part in which they discovered his true body, since his exclamation here was supposed to reflect that part. That’s why it’s very inconsistent.